In modern wavelength division multiplexing (WDM) networks, a loss-of-signal (LOS) condition causes signal power spikes that result in degradation in signal-to-noise ratio (SNR), increase in bit error rate (BER) and damage of downstream optical components. To compensate for unexpected power spikes of optical signals propagating in a WDM network, an electrically controlled variable optical attenuator (eVOA) (coupled with power monitoring and microcontroller apparatus) is typically inserted in the path of an incoming signal for each wavelength. The attenuator's setting is usually adjusted to a pre-determined fixed value which may or may not be sufficient to reduce and/or eliminate the unexpected signal power spikes.
A typical prior art eVOA apparatus includes an eVOA followed by an optical tap coupler for detecting the power of the optical signal at an output of the eVOA. The eVOA and the optical tap coupler are connected to a microcontroller. The microcontroller uses feedback from the optical tap coupler to control the eVOA attenuation to achieve a constant output optical power. In this architecture, a loss-of-signal (LOS) condition is declared when the signal measured at the output of the optical tap coupler drops below a loss-of-signal (LOS) power threshold.
Prior art offers numerous eVOA control mechanisms for handling a loss-of-signal (LOS) condition, wherein the eVOA attenuation is held at a fixed attenuation when a LOS condition is detected. Both U.S. Pat. No. 6,207,949 entitled, “Method and apparatus for stabilizing attenuators in optical networks” to Jackel, J., issued on Mar. 27, 2001, and U.S. Pat. No. 6,304,347 entitled, “Optical power management in an optical network” to Beine, T., et al, issued on Oct. 16, 2001) teach that the eVOA attenuation has to be kept at a constant attenuation that is less than the maximum attenuation of the eVOA. This leads to an exposure to potential disruption or damage of downstream optical components in the event of a sudden power spike in the WDM network, when said constant less than maximum attenuation is not sufficient to attenuate the power spike.
FIG. 1 shows a diagram 100 illustrating an operation and a control problem of a prior art eVOA apparatus when an eVOA attenuation is kept at fixed non-maximum attenuation during a LOS condition. Referring to FIG. 1, graphs (a), (b), and (c) are for an input power versus time, an output power versus time and eVOA attenuation versus time, respectively. In graph (a), at time t1, the input power 10 is removed and then reapplied after a specified time period t2, where the input optical power 115 is much higher. Between times t1 and t2 the input power 112 is zero. This may correspond, e.g., to the cleaning of a dirty patch cable, which is causing unwanted attenuation. In graph (c), before time t1, the eVOA attenuation is set at non-maximum attenuation 153. When the input power 110 in graph (a) is removed (that is at time t1), the eVOA control circuit maintains the eVOA attenuation at non-maximum attenuation 157 between times t1 and t2 and at attenuation 190 between times t2 and t3. The eVOA minimum attenuation (MinAtt) 155 and maximum attenuation (MaxAtt) 175 are shown in graph (c). Referring now to graph (b), before time t1, the output power 163 is a valid optical signal power (that is output signal power above the LOS power threshold). Between times t1 and t2, the output power 167 is dropped below the LOS power threshold 165. Then, at time t2, when the patch cord is reinserted, the output power spike 130 surges significantly above the previous output power 163, as shown in graph (b). This output power spike 130 in graph (b) lasts until the microcontroller detects the presence of optical power and attenuates the eVOA so as to reach the steady state output power 160.
FIG. 2 shows a diagram 200 illustrating limitations of the prior art apparatus that prevent holding the eVOA attenuation at its maximum attenuation. Referring to FIG. 2, graphs (a), (b), and (c) are for an input power versus time, an output power versus time and eVOA attenuation versus time respectively. In graph (a), at time t1, the optical input power 210 is removed and then restored after a specified time period t2, where the input optical power 215 is much higher. Between times t1 and t2 the input power 212 is zero. In graph (c), before time t1, the eVOA attenuation 290 is set at the non-maximum attenuation 295. When the input power 210 in graph (a) is removed (that is at time t1), the eVOA control circuit maintains the eVOA attenuation 290 at the maximum attenuation (MaxAtt) 275 between times t1, t2, and t3. The eVOA minimum attenuation (MinAtt) 255 and maximum attenuation (MaxAtt) 275 are shown in graph (c). Referring now to graph (b), before time t1, the output power 230 is a valid optical signal power (that is output signal power above the LOS power threshold). Between times t1 and t2, the output power 235 is below the LOS power threshold 265, while the eVOA attenuation 290 is set to the maximum attenuation (MaxAtt) 275 in graph (c). Between times t2 and t3, the output power 240 is also below the LOS power threshold 265 and thus, the microcontroller never determines if there is sufficient optical power at the input to the eVOA.
Prior art U.S. Pat. No. 6,304,347 also teaches an apparatus that has an eVOA coupled to two optical tap couplers that are connected to a microcontroller. One optical tap coupler leads the eVOA for detecting the power of the optical signal at an input to the eVOA and another optical tap coupler follows the eVOA for detecting the power of the optical signal at an output of the eVOA. This apparatus allows the microcontroller to monitor the optical signal power at the input to the eVOA and at the output of the eVOA. If a LOS condition is declared when the signal measured at the input to the eVOA, the optical tap coupler drops below a LOS power threshold. This overcomes the problems surrounding the prior art apparatus described above with an optical tap coupler following the eVOA. The eVOA may be kept at a maximum attenuation without risk of failing to detect the presence of the optical signal power, but at the expense of having an additional optical tap and monitoring the signal power at both input and output of the eVOA.
From a cost perspective it is desirable to use the eVOA apparatus with only one optical tap coupler that follows the eVOA to save the cost of another optical tap coupler that is before the eVOA. Having only one optical tap coupler that follows an eVOA also saves physical space and electrical power on the line card where the circuitry is housed.
Unfortunately, none of the existing prior art apparatus provides an effective and reliable operation of eVOA, while minimizing the number of optical components being used.
Accordingly, there is a need for the development of improved methods and apparatus for power control in optical control systems and WDM networks, which would reduce and/or avoid the shortcomings and limitations of the prior art.